1. Field of the Invention
The present invention is related to hydraulic actuators, and more particularly to flow equalization in multiple actuator systems such as automobile lifts and the like.
In lifting large loads such as vehicles, it is desireable to arrange a plurality of lift units at opposite sides of the vehicle for providing virtually unobstructed access to the underside of the vehicle. A variety of means have been utilized for coordinating the operation of the lift units so as to maintain the vehicle level as it is raised and lowered, including the use of a single hydraulic actuator in one of the units, and a mechanical actuator in an opposite unit that is coupled to the hydraulic actuator such as by a chain.
2. Description of the Prior Art
A pair of hydraulic actuators can be synchronized by an interconnecting cable mechanism. Normally, the cable mechanism is only lightly loaded to the extent of an unbalanced loading of the actuators. However, in case of failure of one of the actuators, the cable mechanism must carry the full load of the failed member. U.S. Pat. No. 4,500,071 to Bagwell et al. also discloses the use of master and slave hydraulic actuators in opposing lift units, the master actuator having a dual-chamber cylinder, the slave actuator having a single-chamber cylinder.
It is also known to synchronize a multiple actuator system by the use of hydraulic flow dividers, as disclosed in U.S. Pat. No. 4,475,714 to Heiskell et al., wherein a trio of hydraulic lift actuators for a grand piano are synchronized by a triple gear-pump flow divider that is connected for providing equal volumes of flow under pressure to each of the actuators. Although the synchronization of multiple hydraulic actuators by hydraulic means avoids the problem of having a mechanical mechanism that connects the actuators, such synchronization is not always effective. In automobile lifts, for example, it is customary to provide each lift unit with a safety latch, the safety latches being released when it is desired to lower the load. If hydraulic pressure is released with fewer than all of the latches engaged, or if one of the latches inadvertently remains engaged during an attempted lowering of the vehicle, the actuators can lose synchronization as a result of cavitation of the hydraulic system, with disastrous consequences.
Thus there is a need for flow equalizing multiple hydraulic actuator system that is not subject to cavitation in the event that the equalizer is called open to prevent the retraction of one of its loaded actuators.